When it comes to creatures, there is an important difference in how they are treated in the Heroic design compared to HoMM designs:

Towns no longer have predefined creature and unit line ups. Each town is initially inhabited by a single race of creatures (townsfolk) and the few units that they produce. Other creatures settle in the town as the game progresses, depending on whom the player encounters on the adventure map and is able to befriend. This makes the town creature and unit line up highly situational

Different factions no longer have the same economy, nor is their economy fixed. What resources they need is strongly dependent on what creatures have settled there. The creature line up also determines what structures they are able to build, and thus what resources they need and are able to produce

A clear distinction is made between creatures (eg: Humans) and units (eg: Archers, Swordsmen). The player can choose which units to train creatures into, and is sometimes able to combine multiple creatures, from the same or different races, into a single more powerful unit

Animals: non-sentient creatures that cannot be negotiated with (players can use them as they see fit), cannot serve as units on their own (only in conjunction with other types of creatures), and cannot serve as heroes

Hybrids: semi-sentient creatures that act like sentient creatures in all respects except that they cannot serve as heroes

Sentient creatures: live in dwellings, only serve players if they have signed a treaty or been conquered, can use (ride) animals, can serve as units on their own, and can serve as heroes

The classification would ideally apply to all the creatures in the game, regardless of faction associations. So halflings, humans, elves, dwarves, angels, dragons, etc, would all be sentient creatures.

I mean something like a...tiger. Lions are social beings, tigers not. But applying that to sentient universe. If we, for example, consider a Dragon a non-social sentient being, who won't live with others of their kin, but that can negotiate a understanding and cooperation for survival between other races.

"There’s nothing to fear but fear itself and maybe some mild to moderate jellification of bones." Cave Johnson, Portal 2.

Yes, I see no reason to bar non-social sentient creatures from serving as heroes.

On the subject of heroes, I would like to model them along racial lines, so that higher-level creatures naturally have more powerful heroes. A player would only be able to hire dragon heroes, say, once an external dragon dwelling has allied with his town. This wouldn't apply to campaign heroes, but regular ones that are traditionally hired from a tavern.

Wouldn't have 'stranded' sort of heroes perhaps? For example, a hero we find in a dungeon, sealed, trapped, en-caged, or anything along that line. They would be one of a kind, more likely related to anything of ancient times...however, at the same time, something that old might not be logically to adapt easily to any modern culture after being released, dunno, lol.

"There’s nothing to fear but fear itself and maybe some mild to moderate jellification of bones." Cave Johnson, Portal 2.